Phân tích giao thức mac của mạng LAN không dây IEEE 802.11

International Journal of Computer Networks and Communications Security

C

VOL. 1, NO. 5, OCTOBER 2013, 201–209
Available online at: www.ijcncs.org
ISSN 2308-9830

N

C

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Analysis the MAC Protocol of IEEE 802.11 Wireless LAN
Md. Mizanur Rahman1, Farhana Enam2
1

Department of ICE, University of Rajshahi, Bangladesh

2

Assistant Professor, Department of ICE, University of Rajshahi, Bangladesh
E-mail: ,

ABSTRACT
An ad hoc network is a collection of wireless mobile nodes dynamically forming a temporary network
without the use of any existing network infrastructure or centralized administration. The architecture of the
IEEE 802.11 WLAN is designed to support a network where most decision making is distributed to the
mobile striation. The first, IEEE Standard 802.11a, is an orthogonal frequency domain multiplexing
(OFDM) radio in the UNII bands, delivering up to 54 Mbps data rates. The second, IEEE Standard 802.11b

is an extension to the DSSS PHY in the 2.4 GHz band, delivering up to 11 Mbps data rates. In this Project
study the main emphasis has been given to the MAC layer of the Wireless LAN IEEE 802.11 MAC
Protocol. Also the physical layer of the Wireless LAN IEEE 802.11 MAC Protocol has been described. The
implementation of the Wireless IEEE 802.11 MAC Protocol is observed through a simulation.
Keywords: WLAN, MAC Protocol, CSMA/CD, MAC Frame Format, IEEE802.11.
1

INTRODUCTION

Wireless networking is a fast-growing technology
that allows users to access information and services
electronically, regardless of their geographic
position without cables, Wireless networks can be
classified in two types: Infrastructured Networks
and Infrastructureless Networks.
Infrastructure network makes use of a high speed
wired or wireless network. A mobile host
communicates with a bridge in the network (called
base station) within its communication radius. The
mobile unit can move geographically while it is
communicating. When it goes out of range of one
base station, it connects with new base station and
starts communicating through it. This is called hand
off.
In ad hoc networks, all nodes are mobile and can
be connected dynamically in an arbitrary manner.
All nodes of these networks behave as routers and
take part in discovery and maintenance of routes to
other nodes in the network. Ad hoc networks are
very useful in emergency search-and-rescue

operations, meetings or conventions in which
persons wish to quickly share information, and
acquisition operations in inhospitable terrain [1].

Fig. 1. Pure ad hoc networks

2

IEEE802.11 WIRELESS LAN

Early wireless LAN products, introduced in the
late 1980s, were marketed as substitutes for
traditional wired LANs. A wireless LAN saves the
cost of the installation of LAN cabling and eases
the task of relocation and other modifications to
network structure. However, this motivation for
wireless LANs was overtaken by events. First, as
awareness of the need for LANs became greater,
architects designed new buildings to include
extensive rewiring for data applications. Second,
with advances in data transmission technology,


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there is an increasing reliance on twisted pair
cabling for LANs and in particular, Category 3 and
Category 5. Thus, the use of a wireless LAN to
replace wired LANs has not happened to any great

extent.
However, in a number of environments, there is a
role for the wireless LAN as an alternative to a
wired LAN. Examples include buildings with large
open areas, such a manufacturing plants, stock
exchange trading floors, and warehouses; historical
buildings with insufficient twisted pair and where
drilling holes for new wiring is prohibited; and
small offices where installation and maintenance of
wired LANs is not economical. In all of these cases,
a wireless LAN provides an effective and more
attractive alternative. In most of these cases, an
organization will also have a wired LAN to support
serves and some stationary workstations. For
example, a manufacturing facility typically has an
office area that is separate from the factory floor
but that must be linked to it for networking
purposes. Therefore, typically, a wireless LAN will
be linked into a wired LAN on the same premises.
Thus, this application area is referred to as LAN
extension.
Figure 2 indicates a simple wireless LAN configuration that is typical of many environments. There
is a backbone wired LAN, such as Ethernet, That
supports servers, workstations, and one or more
bridges or routers to link with other networks. In
addition, there is a control module (CM) that acts as
an interface to a wireless LAN. The control module
includes either bridge or router functionality to link
the wireless LAN to the backbone[2]. It includes
some sort of access control logic, such as a polling

or token-passing scheme, to regulate the access
from the end systems. Note that some of the end
systems are standalone devices, such as a
workstation or a server. Hubs of other user modules
(UMs) that control a number of stations off a wired
LAN may also be part of the wireless LAN
configuration.

Fig. 2. Example Single-Cell Wireless LAN
Configuration

The configuration of Figure 2 can be referred to
as a single-cell wireless LAN; all of the wireless
end systems are within range of a single control
module. Another common configuration, suggested
by Figure 3 is a multiple-cell wireless LAN. In this
case, there are multiple control modules intercomnectted by a wired LAN. Each control module
supports a number of wireless end systems within
its transmission range. For example, with an
infrared LAN, transmission is limited to a single
room; therefore, one cell is needed for each room in
an office building that requires wireless support [3].

Fig. 3. Example Multiple-Cell wireless LAN
Configuration


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2.1

Wireless LAN Requirements

 Collocated network operation: As wireless
LANs become more popular, it is quite
likely for two or more wireless LANs to
operate in the same area or in some area
where interference between the LANs is
possible. Such interference may thwart the
normal operation of a MAC algorithm and
may allow unauthorized access to a
particular LAN.

A wireless LAN must meet the same sort of
requirements typical of any LAN, including high
capacity, ability to cover short distances, full
connectivity among attached stations, and broadcast
capability, In addition, there are number of requireements specific to the wireless LAN environment.
The following are among the most important
requirements for wireless LANs.
 Throughput: The medium access control
protocol should make as efficient use as
possible of the wireless medium to
maximize capacity.

 License-free operation: Users would prefer
to buy and operate wireless LAN products
without having to secure a license for the
frequency hand used by the LAN.

 Number of nodes: Wireless LANs may need
to support hundreds of nodes across multiple
cells.
 Connection to backbone LAN: In most
cases, interconnection with stations on a
wired backbone LAN is required. For
infrastructure wireless LANs, this is easily
accomplished through the use of control
modules that connect to both types of LANs.
There may also need to be accommodation
for mobile users and ad hoc wireless
networks.
 Service Area: A typical coverage area for a
wireless LAN has a diameter or 100 to
300m.
 Battery power consumption: Mobile workers
use battery-powered workstations that need
to have a long battery life when used with
wireless adapters. This suggests that MAC
protocol that requires mobile nodes to
monitor access points constantly or engage
in frequent handshakes with a base station
reduce power consumption while not using
the network, such as a sleep mode.
 Transmission robustness and security:
Unless properly designed, a wireless LAN
may be interference prone and easily
eavesdropped. The design of a wireless LAN
must permit reliable transmission even in a

noisy environment and should provide some
level of security from eavesdropping.

 Handoff/roaming: The MAC protocol used
in the wireless LAN should enable mobile
stations to move from one cell to another.
 Dynamic
configuration:
The
MAC
addressing and network management aspects
of the LAN should permit dynamic and
automated addition, deletion, and relocation
of end systems without disruption to other
users.
2.2

Wireless LAN Technology

Wireless LANs are generally categorized according to the transmission technique that is used. All
current wireless LAN products fall into one of the
following categories.
 Infrared (IR) LANs: An individual cell of an
IR LAN is limited to a single room, because
infrared light does not penetrate opaque
walls.
 Spread Spectrum LANs: This type of LAN
makes use of spread spectrum transmission
technology. In most cases, these LANs
operate in the ISM (Industrial, Scientific,

and Medical) bands so that no FCC licensing
is required for their use in the United States.
 Narrowband microwave: These LANs
operate at microwave frequencies but do not
use spread spectrum. Some of these products
operate at frequencies that require FCC
licensing, while others use one of the
unlicensed ISM bands [ 4].


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3

MAC PROTOCOL OF IEEE802.11

The IEEE 802.11 MAC layer covers three functional areas: reliable data delivery, access control,
and security. We look at each of these in turn.
3.1

Reliable Data Delivery

As with any wireless network, a wireless LAN
using IEEE 802.11 physical and MZC layers is
subject to considerable unreliability. Noise,
interference, and other propagation effects result in
the loss of a significant number of frames. Even
with error-correction codes, a number of MAC
frames may not successfully be received. This

situation can be dealt with by reliability
mechanisms at a higher layer, such as TCP.
However, timers used for retransmission at higher
layers are typically on the order of seconds. It is
therefore more efficient to deal with errors at the
MAC level [5]. For this purpose, IEEE 802.11
includes a frame exchange is treated as an atomic
unit, not to be interrupted by a transmission from
any other station. If the source does not receive an
ACK within a short period of time, either because
its data frame was damaged or because the
returning ACK was damaged, the source
retransmits the frame.
Thus, the basic data transfer mechanism in IEEE
802.11 involves an exchange of two frames. To
further enhance reliability, a four-frame exchange
maybe used. In this scheme, a source first issues a
request to send (RTS). After receiving the CTS, the
source transmits the data frame, and the destination
responds with an ACK. The RTS alerts all stations
that are within reception rage of the source that an
exchange is under way; these stains refrain from
transmission in order to avoid a collision between
two frames transmitted at the same time. Similarly,
the CTS alert all stations that are within reception
range of the destination that an exchange is under
way. The RTS/CTS portion of the exchange is a
required function of the MAC but may be disabled.
3.2

Access Control

The 802.11 working group considered two types
of proposals for a MAC algorithm: distributed
access protocols, which, like Ethernet, distribute the
decision to transmit over all the nodes using a
carrier-sense mechanism; and centralized access
protocols, which involve regulation of transmission
by a centralized decision maker. A distributed
access protocol makes sense for an ad hoc network
of peer workstations and may also be attractive in
other wireless LAN configurations that consist
primarily of bursty traffic. A centralized access

protocol is natural for configurations sort of base
station that attaches to backbone wired LAN; it is
especially useful if some of the data is time
sensitive or high priority [6].
The end result for 802.11 is a MAC algorithm
called DFWMAC (distributed foundation wireless
MAC) that provides a distributed access control
mechanism with an optional centralized control
built on top to that. Figure 4 illustrates the
architecture. The lower sub layer of the MAC layer
is the distributed coordination function (DCF).
DCG uses a contention algorithm to provide access
to all traffic. Ordinary asynchronous traffic directly
uses DCF. The point coordination function (PCF) is
a centralized MAC algorithm used to provide
contention function. PCF is built on top of DCF and

exploits features of DCF to assure access for its
users. Let us consider these two sub layers in turn
[8].

Fig. 4. IEEE 802.11 Protocol Architecture

3.3

Distributed Coordination Function

The DCF sub layer makes use of a simple CSMA
(carrier sense multiple access) algorithm. If station
has a MAC frame to transmit, it listens to the
medium. If the medium is idle, the station may
transmit; otherwise the station must wait until the
current transmission is complete before transmiting. The DCF does not include a collision detection
function (i.e, CXSMA/CD) because collision
detection is not practical on wireless network. The
dynamic range of the signals on the medium is very
large, so that a transmitting station cannot
effectively distinguish incoming weak signals from
noise and the effects of its own transmission.
To ensure the smooth and fair functioning of this
algorithm. DCF includes a set of delays that
amounts to a priority scheme. Let us start by
considering [7] a single delay known as an
interframe space (IFS). In fact, there are three
different IFS values, but the algorithm is best
explained by initially ignoring this detail. Using an

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IFS, the rules for CSMA access are as follows
Figure 5.
1)
A station with a frame to transmit senses
the medium. If the medium is idle, it waits to see if
the medium remains idle for a time equal to IFS. If
so, the station may transmit immediately.
2)
If the medium is busy (either because the
station initially finds the medium busy or because
the medium becomes busy during the IFS idle
time), the station defers transmission and continues
to monitor the medium until the current
transmission is over.
3)
Once the current transmission is over, the
station delays another IFS. If the medium remains
idle for this period, then the station backs off a
random amount of time and again senses the
medium. If the medium is still idle, the station may
transmit. During the back of time, if the medium
becomes busy, the backoff timer is halted and
resumes when the medium becomes idle.

To ensure that backoff maintains stability, a
technique known as binary exponential backoff is

used. A station will attempt to transmit repeatedly
in the face of repeated collisions, but after each
collision, the mean value of the random delay is
doubled. The binary exponential backoff provides a
means of handling a heavy load. Repeated failed
attempts to transmit result in longer and longer
backoff time, which helps to smooth out the load.
Without such a backoff, the following situation
could occur. Two or more stations attempt to
transmit at the same time, causing a collision.
These stations then immediately attempt to
retransmit, causing a new collision.
The preceding scheme is refined for DCF to
provide priority-based access by the simple
expedient of using three values for IFS.


SIFS (short IFS): The shortest IFS, used
for all immediate response actions, as
explained in the following discussion.



PIFS (point coordination function IFS): A
midlength IFS, used by the centralized
controller in the PCF scheme when issuing
polls.



DIFS (distributed coordination function
IFS): The longest IFS, used as a minimum
delay for asynchronous frames contending
for access.

Figure 6 a illustrated the use of these time values.
Consider first the SIFS. Any station using SIFS to
determine transmission opportunity has, in effect,
the highest priority, because it will always gain
access in preference to a station waiting an amount
of time equal to PIFS or DIFS. The SIFS is used in
the following circumstances:

Fig. 5. IEEE 802.11 Medium Access Control Logic

(a) Basic access method


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frames, but not all fields are used in all contexts.
The fields are as follows:
octets 2 2
FC

6
D/I

6

Ad
dre
ss

6 2 6 0 to 2312 4

Add
ress

Add
ress

S
C

Add
ress

Fra
me
bod
y

CRC

FC= Frame control
D/I= Duration/Connection ID
SC= Sequence control
(b) PCF superframe construction

(a) MAC frame

Fig. 6. IEEE 802.11 MAC Timing

bits
 Acknowledgment (ACK): When a station
receives a frame addressed only to itself (not
multicast or broadcast) it responds with an
ACK frame after waiting only for an SIFS
gap. This has two desirable effects. First,
because collision detection is not used, the
likelihood of collisions is greater than with
CSMA/CD, and the MAC-level ACK
provides for efficient collision recovery.
Second, the SIFS can be used to provide3
efficient delivery of an LLC protocol data
unit (PDU) that requires multiple MAC
frames. In this case, the following scenario
occurs. A station with a multiframe LLC
PDU to transmit sends out the MAC frames
one at a time. The recipient acknowledges
each frame after SIFS. When the source
receives an ACK, it immediately (after
SIFS) sends the next frame in the sequence.
The result is that once a station has
contended for the channel, it will maintain
control of the channel until it has sent all of
the fragments of an LLC PDU.
 Clear to Send (CTS): A station can ensure
that its data frame will get through by first

issuing a small Request to Send (RTS)
frame. The station to which this frame is
addressed should immediately respond with
a CTS frame if it is ready to receive. All
other stations receive the RTS and defer
using the medium.
 Poll response: This is explained in the
following discussion of PCF.
Figure 7 s shows the 802.11 frame format. This
general format is used for all data and control

2
Prot
ocol
Vers
ion

2
Ty
pe

4

1
Subt
ype

T
o
D

S

1

1

1

1

1 1 1

Fr
om
DS

M
F

R
T

P
M

M
D

W

O

DS= Distribution System
MD= More data
MF= More fragments
W= Wired equivalent privacy bit
RT= Retry
O= Order
PM= Power management
(b) Frame control field
Fig. 7. IEEE 802.11 MAC Frame Format

 Frame control: Indicates the type of frame
and provides control information, as
explained presently.
 Duration/connection ID: If used as a duration field, indicates the time (in microseconds) the channel will be allocated for
successful transmission of a MAC frame. In
some control frames, this field contains an
association, or connection, identifier.
 Addresses: The number and meaning of the
address fields depend on context. Address
types
include
source,
destination,
transmitting station, and receiving.
 Sequence control: Contains a 4-bit fragment
number subfield, used for fragmentation and
reassembly, and a 12-bit sequence number
used to number frames sent between a given

transmitter and receiver.
 Frame body: Contains an MSDU or a
fragment of an MSDU. The MSDU is a LLC
protocol data unit or MAC control
information.


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M. M. Rahman and F. Enam / International Journal of Computer Networks and Communications Security, 1 (5), October 2013

includes the AP (access point). Its purpose
is to request that the AP transmit a frame
that has been buffered for this station while
the station was in power-saving mode.

 Frame check sequence: A 32-bit cyclic
redundancy check.
The frame control field, shown in Figure 6.4b,
consists of the following fields:
 Protocol version: 02.11 version, currently
version 0.
 Type: Identifies the frame as control,
management, or data.
 Subtype: Further identifies the function of
frame. The 14.3 Defines the valid
combinations of type and subtype.
 To DS: The MAC coordination sets this bit
to 1 in a frame destined to the distribution
system.
 From DS: The MAC coordination sets this

bit to1in a frame leaving the distribution
system.

 Request to send (RTS): The station sending
this message is alerting a potential destination, and all other stations within reception
range, that it intends to send a data frame to
that destination.
 Clear to send (CTS): This is the second
frame in the four-way exchange. It is sent by
the destination station to the source station
to grant permission to send a data frame.
 Acknowledgment: Provides an acknowledgment from the destination to the source that
the immediately preceding data, management, PS-Poll frame was receive correctly.

 More fragments: Set to 1 if more fragments
follow this one.

 Contention-free (CF)-end: Announces the
end of a contention-free period that is part of
the point coordination function.

 Retry: Set to 1 if this is a retransmitting
station is in a sleep mode.

CF-end+CF-ack: Acknowledges the CF-end. This
frame ends the contention-free period and releases
from the restrictions associated with that period.

 Power management: Set to if
transmission station is in a sleep mode.

3.4.2 Data Frames

the

 More data: Indicates that a station has
additional data to send. Each block of data
may be sent as one frame or a group of
fragments in multiple frames.
 WEP: Set to if the optional wired equivalent
protocol is implemented. WEP is used in the
exchange of encryption keys for secure data
exchange.
 Order: Set to 1 any data frame sent using the
Strictly Ordered service, which tells the
receiving station that frames must be
processed in order [9].

There are eight data frame subtypes, organized
into two groups. The first four subtypes define
frames that carry upper-level data from the source
station to the destination station. The four datacarrying frames are as follow:


Data: This is the simplest data frame. It
may be used in both a contention period
and a contention-free period.



Data + CF-Ack: May only be sent during a
contention-free period. In addition to
carrying data, this frame acknowledges
previously received data.



Data + CF-Poll: Used by a point
coordinator to deliver data to a mobile
station and also to request that the mobile
station send a data frame that it may have
buffered.



Data + CF-Ack + CF-Poll: Combined the
functions of the Data + CF-Ack and CFPoll into a single frame.

3.4
Various MAC frame types
3.4.1 Control Frames
Control frames assist in the reliable delivery of
data frames. There are six control frame sub types:
 Power save-poll (PS-Poll): This frame is
sent by any station to the station that


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3.4.3 Management Frames

 Authentication: Multiple authentication
frames are used in an exchange to authenticcate one station another, as described
subsequently.

Management frames are used to manage
communications between stations and APs. The
following subtypes are included:
 Association request: Sent by a station to an
AP to request an association with this BSS.
This frame includes capability information,
such as whether encryption is to be used and
whether this station is pollable.
 Association response: Returned by the AP to
the station to indicate whether it is accepting
this association request.
 Reassociation request: Sent by a station
when it moves from one BSS to another and
needs to make an association with the AP in
the new BSS. The station uses reassociation
rather than simply association so that the
new AP knows to negotiate with the old AP
for the forwarding of data frames.
 Reassociation response: Returned by the AP
to the station to indicate whether it is
accepting this reassociation request.

 Deauthentication: Sent by a station to
another or AP to indicate that it is terminating secure communications [10].

4

SIMULATION RESULT

Figures should be labeled with A Simulation
Environment created using the programming
language Matlab-7 was studied to observe the IEEE
802.11 MAC protocol implementation.
Various parameters of an ad hoc network
employing IEEE-802.11 protocol such as number
of movable nodes, their data transmission range,
node mobility, data transmission duration was
varied to find out the performance of the MAC
protocol.
In this simulation the data packet transmission
between various nodes has been shown through
solid red line and the acknowledgement data
transmission is shown using solid green lines as
shown in the following Figure:

 Probe request: Used by a station to obtain
information from anther station or AP. The
frame is used to locate an IEEE 802.11 BSS.
 Prove response: Response to a probe
request.
 Beacon: Transmitted periodically to allow
mobile stations to locate and identify a BSS.
 Announcement traffic indication message:
Sent by a mobile station to alert other
mobile stations that may have been in low

power mode that this station has frames
buffered and waiting to be delivered to the
station addressed in this frame.
 Dissociation: Used by a station to terminate
an association.

Fig. 8. Data and ACK packet transmission between
source and destination
And also the RTS and CTS data transmission are
also shown through dotted red and green lines
respectively, which is shown below:


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Fig. 9. Data, ACK, RTS and CTS packet transmission
between source and destination

5

CONCLUSION

Through study and also simulation it is found that
the IEEE 802.11 standard quite efficiently
overcomes the collisions of control packets. It can
also reduce the hidden terminal problem and it can
also handle the exposed terminals problems to some
extent.
From the Ad hoc network simulation study it is

found that IEEE 802.11 MAC Protocol can quite
efficiently establish Wireless communication link
among various nodes when the number of nodes
transmission range, nodes mobility, data
transmission duration are varied.
The goals of the IEEE 802.11 standard is to
describe a WLAN that delivers services previously
found only in wired networks, e.g., high
throughput, highly reliable data delivery, and
continuous network connections. In addition, IEEE
802.11 describes a WLAN that allows transparent
mobility and built-in power saving operations to the
network user.
6

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